Recirculation structure for a turbocompressor

ABSTRACT

A recirculation structure for turbo-compressors has an annular chamber that is positioned in the area of the free blade ends of a rotating blade ring and that radially borders the main flow channel, and has a multitude of guide vanes arranged in the annular chamber and distributed around its circumference. The annular chamber enables the passage of air flow in the forward and/or rear areas, and the guide vanes are firmly fixed to at least one wall of the annular chamber and otherwise are designed to be free-standing. The tips of the guide vanes that face the annular chamber extend along and/or near the contour of the main flow channel, and axially overlap the free blade ends or axially border the area of the free blade ends.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to a recirculation structure forturbo-compressors, a turbo-compressor, an aircraft engine, and astationary gas turbine.

Recirculation structures for turbo-compressors have been known in theart for quite a long time, and in the trade are generally referred to as“casing treatments”. Their primary function is to increase theaerodynamically stable operating range of the compressor, wherein theso-called surge margin is shifted to higher compressor pressures, i.e.to a higher compressor load. The failures that are responsible forlocalized stalling and ultimately for the surging of the compressoroccur on the casing side at the ends of the rotating blades of one ormore compressor stages, and on the hub side at the ends of the vanesthat lie radially inside, because in these areas the aerodynamic load isthe highest. By recirculating the “air particles” that circulate betweenthe blade tips at blade speed, and whose energy level is reduced, intothe main stream with an increase in energy, the flow in the area of theblade ends is again stabilized. Because flow disruptions as a rule donot occur evenly over all the stages, in terms of fluid mechanics, acircumferential balancing, in addition to the essentially axialrecirculation, should also be possible. The primary disadvantage ofknown “casing treatments” is that, although they do increase the surgemargin, they also reduce the efficiency level of the compressor.

German publication DE 33 22 295 C3 protects an axial fan with a “casingtreatment”. Recognizable therein is an annular chamber (8) in whichguide vanes (9) are fixed. In the downstream area over the ends of therotating blades is an area that is open circumferentially into which theguide vanes do not extend. Characteristic of this type of “casingtreatment” is a closed ring (7) that is aligned generally with the shapeof the main flow channel, with the ring separating the rear intake areafrom the forward outlet area of the recirculation structure, and forminga smooth, closed surface area.

A quite similar “casing treatment” is known from German publication DE35 39 604 C1, wherein an area that is open circumferentially is presentin the forward and rear areas of the annular chamber (7). The radiallyinside ring 6 is also seen here.

A more recent “casing treatment” is known from U.S. Pat. No. 5,282,718.Here, the annular chamber (18, 28) and the guide vanes (24) are improvedin terms of fluid mechanics. Here again, the intake and outlet of therecirculation flow are separated by a solid ring that is smooth andclosed on the side of the blades. Rings of this type in the blade areamust ordinarily be provided with a contact or intake coating in casethey should come in contact with the blade tips.

Further “casing treatments” having axial or axially angled grooves aredisclosed e.g. in U.S. Pat. No. 5,137,419. These are not taken intoconsideration here, however, because the grooves are not connected toone another in these versions; hence no circumferential flow comparisonis possible.

U.S. Pat. No. 4,511,308 relates to ventilators (fans, blowers) havingvarious “casing treatment” designs. The simplest design according toFIG. 6 possesses only one annular chamber without guide vanes. In theembodiments according to FIGS. 1 and 3, guide vanes are mounted in theannular chamber, and the upstream casing wall (22) is extended beyondthe radial inside edges of the guide vanes (21) like a cylindrical orconical socket piece, so that on the upstream, forward end of theannular chamber no outlet of the recirculation flow into the main flowis possible. FIG. 5 shows guide vanes (21) that are mounted on theforward end wall and on the outer circumference of the annular chamber,and are further designed to be freestanding. Here there is no socket- orring-like element that connects or covers the guide vanescircumferentially. The free, radially inside edges of the guide vanes(21) rise from the front to the back from the diameter of the intakecasing (15) up to the greatest diameter of the annular chamber (16). Inthis manner, although in the downstream area the guide vanes axiallyoverlap the upstream area of the blade ends (14), due to the greatradial distance between the guide vanes (21) and the blade ends (14), noeffective and defined guidance of the recirculation air is possible. Offurther disadvantage is the large volume of the annular chamber (16) inrelation to the blade dimensions. This type of embodiment is neitheraerodynamically nor constructively suitable for a turbo-compressor.

In view of the disadvantages of the state of the art solutions, theobject of the invention is to provide a recirculation structure forturbo-compressors that will enable a substantial enhancement of thesurge margin, and thus a clear expansion of the stable operating range,without a significant reduction of the efficiency of the compressor.

The essence of the invention is to have the tips of the guide vanes thatface the annular chamber lie on or near the contour of the main flowchannel, and axially overlap the free ends of the blades or axiallyborder the area of the free ends of the blades. In this manner, annularelements with contact coatings, etc. can be eliminated. The above-citedpatent specifications show that up to now the professional world hasconsistently tried to design recirculation structures that will besmooth, without gaps, and closed over the largest possible axial range,up to the main flow channel, i.e. up to the so-called annular chamber,in order to effect an extension of the contours of the main flow channelthat will be as favorable in terms of flow and loss as possible. Theinvention, in contrast, leads to gaps, fissured surfaces, etc., and thuswould appear to be disadvantageous and inexpedient. Test have shown,however, that the recirculation structure of the invention is superiorto known solutions in terms of both enhancing the surge margin andimproving the level of efficiency. This can be explained in aerodynamicterms in that the free, informal design of the recirculation flow in theopen annular chamber with free-standing guide vanes and circumferentialflow links is more important than the greatest possible gap-freeextension of the contour of the main flow channel. The absence of aclosed ring has the further advantages that no contact or intake coatingof the guide vanes is necessary, and radial space and weight are saved,resulting in structural mechanical advantages. However, a definedcontrol of the recirculation flow—without annular elements—is achievedonly if the free edges of the guide vanes run relatively close to theedges of the blades, partially overlapping them axially, or at leastlying adjacent to their space. Only in this way can a compact “casingtreatment” that is suitable for use in a compressor ultimately beachieved.

The ratio of the axial length of the annular chamber to the axial lengthof the blade ends preferably is 0.2 to 1.5. In the case of wide bladeshaving a large axial span at the blade end, the ratio will be closer to0.2; with narrow blades having a small axial span at the blade end, theratio will be closer to 1.5.

In the preferred embodiment, the ratio of the radial height to the axiallength of the annular chamber is 0.1 to 1.0. With aircraft engineshaving very strict standards in terms of space requirements, contours,etc., attempts will be made to manage with a smaller ratio, i.e. asmaller radial height. For stationary applications having adequate spaceavailable it is possible to go closer to the upper limit. With axiallyshort annular chambers one would also tend more to approach the upperlimit.

It is further preferred that the tips of the guide vanes that facetoward the annular chamber are radially recessed, at least in the areaof the free blade ends, far enough that during normal operation the endsof the blades will not come into contact with the guide vanes. This isdue to the fact that the tips of the blades may be damaged by brushingagainst something, especially against hard, inflexible guide vane tips.The recessing of the guide vane tips is not in contradiction to therequirement that the tips should lie on or near the contour of the mainflow channel, because the small radial gap dimensions required toprevent contact are practically without consequence in terms of fluidmechanics, i.e. they do not negatively affect recirculation.

Preferred embodiments of the recirculation structure, as well as aturbo-compressor, an aircraft gas turbine, and a stationary gas turbineall form the subject matter of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to thedrawings.

FIG. 1 shows a partial longitudinal section of a compressor in an axialconstruction, in the area of a recirculation structure on the side ofthe casing,

FIG. 2 shows a comparable partial longitudinal section in the area of arecirculation structure on the side of the hub,

FIG. 3 shows a partial cross-section through the recirculation structureshown in FIG. 1,

FIG. 4 shows a partial view of the recirculation structure shown inFIGS. 1 and 3, radially from the inside,

FIG. 5 shows a partial longitudinal section in the area of arecirculation structure on the side of the casing, modified from thatshown in FIG. 1,

FIG. 6 shows a partial longitudinal section in the area of arecirculation structure on the side of the casing, modified from thoseshown in FIG. 1 and FIG. 5, and

FIG. 7 shows a further partial longitudinal section in the area ofanother modified recirculation structure on the side of the casing.

DETAILED DESCRIPTION OF THE INVENTION

The recirculation structure 1 according to FIG. 1 is integrated into thecasing 5 of a turbo-compressor, and thus is to be referred to as a“casing treatment”. The direction of flow in the bladed main flowchannel 9 is indicated on the left by an arrow, thus it flows from leftto right. In the area shown here, the flow first strikes a vane ring 13,then a rotating blade ring 20, and finally another vane ring 14. Theradial outer contour 11 of the main flow channel 9 corresponds to theinner contour of the casing 5, and for purposes of clarification iscontinued here by dotted/dashed lines to the left and right of theactual illustration. The static recirculation structure 1 operates inconjunction with the rotating blade ring 20, and lies for the most partaxially in front of said blade ring, i.e. upstream. The annular chamber29, which together with the guide vanes 37 forms the recirculationstructure 1, borders the main flow channel 9 radially from the outside,and is open toward it. The tips 41 of the guide vanes 37 lie on or nearthe contour 11 of the main flow channel 9, i.e. they are aligned atleast nearly with the inner contour of the casing. The guide vanes 37may be comprised of a metal, such as a Ni-based alloy, or of a lightmetal, such as Al, or of a plastic, such as thermoplastics,thermosetting plastics, or elastomers. The forward wall 33 and the rearwall 34 of the annular chamber 29 are tilted forward beginning at theirradial inner edges 35, 36, in order to make them favorable in terms ofrecirculation flow, as indicated by a small arrow.

The angle of slope of the forward wall is indicated by a, and may beequal to or different from the angle of the rear wall 34. Between theforward wall 33, the guide vanes 37, and the rear wall 34 are recesses45, 46, which permit flow processes inside the annular chamber in acircumferential direction, in addition to the predominantly axial flowof recirculation. The free blade ends of the rotating blade ring 20, inthe area of which flow disruptions occur most frequently, are indicatedby the number 25. The axial overlapping of the guide vanes 37 and theblade ends 25 is indicated by UE 1.

In contrast to FIG. 1, FIG. 2 shows a recirculation structure that isintegrated into a rotating hub 8. In the main flow channel 10 from leftto right are visible a rotating blade ring 21, a vane ring 15 withradial inner, free blade ends 26, and a rotating blade ring 22. Such anew design for a recirculation structure would consequently be referredto as a “hub treatment”. The recirculation structure 2 comprised of anannular chamber 30 and guide vanes 38, with forward and rear recesses47, 48 between the forward and rear walls of the annular chamber 30 andthe guide vanes distanced from said walls, operates in conjunction witha vane ring 15 that lies primarily downstream. Because here the “hubtreatment” rotates and the vane ring 15 remains stationary, the rotorspeed acts entirely as a differential speed. The mode of operation doesnot differ in principle from that of a “casing treatment”. In aturbo-compressor, “casing treatment” and “hub treatment” can even becombined and used in several stages. The radial inner contour 12 of themain flow channel here corresponds to the outer contour of the hub 8. UE2 is the axial overlapping of the guide vanes 38 with the blade ends 26of the vane ring 15. The guide vanes 38 are rounded at the crossover tothe walls of the annular chamber 30, to increase stability.

FIG. 3 shows a detail from FIG. 1, in cross-section. The guide vanes 37are tilted radially at an angle β such that the blade ends 25 of therotating blade ring 20 force the recirculation flow into the annularchamber 29 without heavy losses, taking into account the direction ofrotation (see arrow). The angle of slope β can decrease from the radialinside to the outside, to the value “zero”, with appropriately curvedguide vanes.

A radial arrangement of the guide vanes, i.e. β=0°, is possible, butless favorable in terms of flow.

The view of FIG. 3 shown in FIG. 4 shows the blade profiling of therotating blade ring 20 in connection with its direction of rotation(arrow), and provides a clear presentation of the profiling andcurvature of the guide vanes 37 that are favorable in terms of flow. Anexpert in the field would recognize that the recirculation outlet wouldtake place in the area of the upstream edge 35 of the annular chamber 29in relation to the rotating blade ring 20, here with counter-swirl. Thedownstream edge of the annular chamber is indicated by the number 36.

The recirculation structure 3 shown in FIG. 5 is a “casing treatment”with an annular chamber 31 integrated into a casing 6. The guide vanes39 here extend up to the forward wall of the annular chamber 31; in therear area are recesses 49, in the immediate area around the blade ends27 of the rotating blade ring 23. UE 3 indicates the axial overlappingof the guide vanes 39 with the blade ends 27. LR is the axial length ofthe annular chamber 31, and HR is its radial height. LS indicates theaxial length of the blade ends 27. The ratio of LR to LS should be 0.2to 1.5; the ratio of HR to LR is 0.1 to 1.0. The tips 43 of the guidevanes 39 here are displaced radially toward the outside in the area ofrotation of the blade ends 27, in order to prevent them from coming intocontact with one another. The vane rings are indicated by the numbers 16and 17.

The recirculation structure 4 shown in FIG. 6 with the annular chamber32 and the guide vanes 40 is also a “casing treatment”, which isintegrated into a casing 7 and operates in conjunction with a rotatingblade ring 24. In contrast to FIG. 5, here the guide vanes 40 extend upto the rear wall of the annular chamber 32. Recesses 50 here areprovided in the forward area due to the guide vanes 40 that aredistanced from the forward wall of the annular chamber 32. The tips 44of the guide vanes 40 extend up to the area of rotation of the bladeends 28.

The recirculation structure 51 shown in FIG. 7 differs from thepreviously described embodiments in that for circumferential flowprocesses in the annular chamber 53, openings 58, 59 that are borderedon all sides are present in the forward and rear areas of the guidevanes 56. There thus remain bridges 60, 61 that increase stability, viawhich the guide vanes 56 are firmly connected to the forward and rearwalls 54, 55 of the annular chamber 53 in the casing 52. Alternatively,an opening 58 or 59 can be replaced by a recess as described above,based upon a distancing of the guide vanes from the wall of the annularchamber 53. It is understood that only one opening 58 or 59 can beprovided in the forward or rear area of the guide vanes.

For all embodiments of the recirculation structure, the tips 41 through44 and 57 of the guide vanes 37 through 40 and 56 need not necessarilybe displaced radially toward the outside if the guide vanes are made ofa soft light metal or a plastic, as contact with the blade ends 25through 28 can then be permitted without damage to the blades.

1. A recirculation structure for a turbo-compressor, comprising: anannular chamber oriented concentrically with a compressor axis in anarea of free blade ends of a rotating blade ring, an axial center of theannular chamber lying in front of an axial center of the free bladeends, said annular chamber radially bordering a contour of a main flowchannel, and a multitude of guide vanes arranged in the annular chamberand distributed around its circumference, wherein the annular chamberpermits flow to pass through circumferentially in at least one offorward and rear areas thereof, wherein the guide vanes are connected toat least one wall of the annular chamber and are free-standing, whereintips of the guide vanes face toward the main flow channel, and are atleast nearly aligned with the contour of the main flow channel, whereinthe tips of the guide vanes axially overlap or axially border the areaof the free blade ends, and wherein the circumferential flow passesthrough openings in forward areas, in rear areas, or in both forward andrear areas of the guide vanes.
 2. A recirculation structure for aturbo-compressor, comprising: an annular chamber oriented concentricallywith a compressor axis in an area of free blade ends of a rotating bladering, an axial center of the annular chamber lying in front of an axialcenter of the free blade ends, said annular chamber radially bordering acontour of a main flow channel, and a multitude of guide vanes arrangedin the annular chamber and distributed around its circumference, whereinthe annular chamber permits flow to pass through circumferentially in atleast one of forward and rear areas thereof, wherein the guide vanes areconnected to at least one wall of the annular chamber and arefree-standing, wherein tips of the guide vanes face toward the main flowchannel, and are at least nearly aligned with the contour of the mainflow channel, wherein the tips of the guide vanes axially overlap oraxially border the area of the free blade ends, and wherein thecircumferential flow passes through at least one of recesses in theguide vanes and openings in the guide vanes in forward or in forward andrear areas of the annular chamber.